The present invention relates to a semiconductor light emitting device to be used as a light source of, for example, illuminations, information display apparatuses and information transmission apparatuses and relates to a manufacturing method therefor.
Conventionally, it has been very important for light emitting diodes (herein below referred to as LEDs) to enhance efficiency to take out internally generated light, i.e., external emission efficiency.
In order to enhance the external emission efficiency of LEDs, LED substrates transparent to emission wavelengths are generally used.
When a substrate opaque to emission wavelengths is used for an LED, the substrate absorbs emitted light and therefore light is emitted substantially only to a face (hereinafter, referred to as an upper face) on one side other than the substrate side with respect to an emission layer.
On the other hand, when a substrate transparent to emission wavelengths is used for an LED, light can be emitted not only from the upper face but also from other faces. Moreover, in the case where the substrate-side face of the LED (hereinafter, referred to as a lower face) is bonded, light going from the emission layer to the substrate side can be reflected by the lower face so as to be emitted from the upper face and lateral faces and the like.
LEDs having such a transparent substrate have conventionally been applied to infrared LEDs with use of InGaAsP-base semiconductor materials, infrared and red LEDs with use of AlGaAs-base semiconductor materials, yellow LEDs with use of GaAsP-base semiconductor materials, green LEDs with use of GaP-base semiconductor materials, and the like.
In recent years, in the development of red, yellow and green LEDs with use of AlGaInP-base semiconductor materials, a wafer bonding technology for directly bonding a plurality of substrates to one another is rapidly coming into practical use. With the wafer bonding technology, substrates transparent to emission wavelengths are bonded to LED substrates so as to enhance the external emission efficiency of the LEDs.
A first prior art of this kind is disclosed in JP No. 3230638 B. In the first prior art, a GaP (gallium phosphorus)-base transparent substrate is directly bonded, through pressurization and high temperature treatment, to the surface of an AlGaInP (aluminum gallium indium phosphorus)-base semiconductor layer formed on a GaAs (gallium arsenide) substrate.
A second prior art is disclosed in JP No. 3532953 B. In the second prior art, an LED emission layer and a transparent layer are wafer-bonded via a bonding layer containing In (indium).
A third prior art is disclosed in JP 2001-53056 A. In the third prior art, first on a first epitaxial layer grown on a first substrate, a second epitaxial layer is grown via a mask, and on the second epitaxial layer, a trench reaching the mask is formed. Next, after a second substrate is wafer-bonded onto the second epitaxial layer, an etchant is put into the trench so as to etch the mask away. By this, the second epitaxial layer and the second substrate are separated from the first substrate and the first epitaxial layer.
Moreover, a fourth prior art is disclosed in JP 2001-57441 A. In the fourth prior art, a trench is formed on at least one of the bonding faces of a first semiconductor substrate and a second semiconductor substrate, before the first semiconductor substrate and the second semiconductor substrate are bonded together.
However, the respective prior arts have following problems.
That is, in the first prior art, it is difficult to uniformly bond the entire surface of a transparent substrate to a wafer with a diameter of 2 inches or 3 inches, which is generally used in manufacturing of LEDs, with an excellent yield.
In a test conducted by the present applicant, with a use of a jig 50 as shown in the schematic front view of FIG. 11A and the schematic plan view of FIG. 11B, a second wafer 123 that was a GaP transparent substrate was pressurized in tight contact to a first wafer 122 composed of a GaAs substrate and an AlGaInP-base semiconductor layer formed on the GaAs substrate, and was subjected to high temperature treatment in a heating furnace. Herein, the first and second wafers 122, 123 were both wafers with a diameter of 2 inches.
When the first and second wafers 122, 123 were taken out of the heating furnace after the high temperature treatment, the first and second wafers 122, 123 had cracks, and therefore the next manufacturing step could not be taken.
FIG. 12A is a schematic plan view showing the first wafer 122 before bonding, and FIG. 12B is a schematic cross sectional view as viewed from the line XIIB-XIIB in FIG. 12A.
FIG. 13A is a schematic plan view showing the first and second wafers 122, 123 after bonding, and FIG. 13B is a schematic cross sectional view as viewed from the line XIIIB-XIIIB in FIG. 13A. It is to be noted that bonded portions are hatched in FIG. 13A.
As shown in FIGS. 13A and 13B, a crack 112 is generated in the first and second wafers 122, 123, and bonded portions 110 are generated at central and radially outer portions of the wafer in an island-like state and so those other than the bonded portions are not bonded. As a result, a bonding failure occurs.
Therefore, the first prior art has a problem of difficulty of its application to mass production of LEDs.
In the second prior art, after the LED layer is formed on the grown substrate and before the transparent substrate is wafer-bonded, the grown substrate is removed. The LED layer after the grown substrate has been removed is thin and prone to breakage, and this causes reduction in yield.
Further, in the second prior art, in order to suppress breakage and cracking of the wafer during wafer bonding, a unit to pressurize the wafer by a pneumatic piston upon arrival of the wafer at a temperature at which the wafer is softened is necessary. This causes complication of manufacturing equipment and complication of the manufacturing equipment control.
The third prior art does not provide details of the wafer bonding step.
In the fourth prior art, according to a test conducted by the present applicant, with a use of a jig 50, grooves with a width of 30 μm and a depth of 30 μm were formed by dicing at intervals of 280 μm on the surface of a 270 μm-thick first wafer 122 composed of a GaAs substrate and an AlGaInP-base semiconductor layer formed thereon, and a second wafer 123 that was a GaP transparent substrate was pressurized in tight contact to the first wafer 122 and was subjected to high temperature treatment in a heating furnace. In this case, the first and second wafers 122, 123 were both wafers with a diameter of 2 inches.
When the first and second wafers 122, 123 were taken out of the heating furnace after the high temperature treatment, the wafers sometimes had cracks along the groove forming direction. For example, in the case where a groove extending in a direction parallel to <110> direction and a groove extending in a direction perpendicular to the <110> direction were formed on the surface of the first wafer 122, the first and second wafers 122, 123 were broken into about 10 pieces and became useless as a product.